1. Field of the Invention
The present disclosure relates generally to thrust and journal bearings, and in particular, to hydrostatic bearings.
2. Description of the Related Art
Fluid bearings are bearings that operate with a layer of fluid, such as a gas or a liquid, between moving parts. In comparison to conventional bearings such as roller bearings or ball bearings, for example, fluid bearings provide significant reduction in friction and wear. One common type of fluid bearing is the hydrostatic bearing, in which a rotor element is supported by a fluid to rotate relative to a stator element. Typically, the bearing is provided with a fluid supply under pressure to one or more cavities, sometimes referred to as pads, which are commonly formed in a stator element, between the rotor and stator elements. When the total surface force in the cavities balances the downward force of the rotor element, the element lifts off the surface of the stator element, so that the rotor floats on the fluid. This eliminates mechanical contact between the rotor and stator, allowing the rotor to rotate virtually without friction. This condition is referred to herein as full hydrostatic operation.
The surface force is a function of the surface area of the pad and the pressure (psi) of the fluid in the pad. If the surface force drops below the balance force, the rotor will make contact with the stator, possibly resulting in damage to one or both surfaces. If fluid supply pressure is greater than a pressure necessary to establish the balance force, the rotor element is separated further and fluid escapes from the pads, while the surface force remains substantially constant, and equal to the balance force. It is common to maintain a slight overpressure of the fluid supply to ensure that there is no contact between the rotor and stator. However, any excess supply pressure results in loss of fluid. Because of the energy cost associated with pressurizing the fluid in the first place, this loss of fluid represents a loss of energy and a reduction in economy, so such losses are minimized wherever possible.
Several designs have been proposed for the deployment of hydrostatic bearings in hydraulic machines such as pump/motors. However, because of the limitations of hydrostatic bearings, there are problems associated with such use. In an application where the load on the bearing varies, such as in a variable-angle pump/motor, it is important that the fluid supply pressure be sufficiently high that at maximum load levels, the surface force is adequate to maintain the balance force, to avoid damage to the bearing. However, this means that when load levels drop, a significant overpressure will exist, resulting in loss of fluid. While many of the proposed designs attempt to address this problem, they are, for the most part, impractical or ineffective.
FIGS. 1A-1C show sectional views of a portion of a bent-axis pump/motor 100 according to known art. The motor 100 includes a valve plate 102 and a cylinder barrel 104, having a plurality of cylinders 106 within which pistons 108 travel reciprocally. Each of the pistons 108 engages a respective socket formed in a drive plate 110. The drive plate 110 is coupled to an output shaft 120 that is rotationally driven by the motor 100. The drive plate 110 bears against a thrust bearing 118 configured to permit free rotation of the drive plate 110 and shaft 120, while holding the drive plate in position against radial and axial forces acting thereon. A radial bearing 119 is positioned on the shaft 120 to stabilize the shaft while permitting free rotation. The bearing 118 is shown as a combination bearing, configured to bear radial and axial loads. Many motors employ separate axial and radial load bearings.
The cylinder barrel 104 is configured to rotate around a first axis A. The drive plate 110 rotates around an axis B, and is coupled to the rotating cylinder barrel 104 by a constant velocity joint 116 (only portions of which are shown in FIGS. 1A-1C). Accordingly, the cylinder barrel 104 and the drive plate 110 rotate at a common rate.
The valve plate 102, barrel 104, and pistons 108, which define axis A, are configured to rotate with respect to the drive plate 110, which defines axis B, for the purpose of varying the displacement volume of the pump/motor 100. The degree of rotation of axis A away from a coaxial relationship with axis B is typically referred to as the stroke-angle of the device.
When the motor 100 is operating in a motor mode, high-pressure fluid is valved into each cylinder 106 as it passes top-dead-center (TDC). The high-pressure fluid applies a driving force on the face of the piston 108, which acts axially on the piston 108 with respect to axis A. This force is transferred by the piston 108 to the drive plate 110. As each piston 108 passes bottom-dead-center (BDC), the fluid is vented from the piston 106, which allows the piston to be pushed back into the cylinder as the barrel rotates it back toward TDC.
Referring to FIG. 1A, it may be seen that the driving force on the pistons 108 is axial, relative to axis A, but includes both axial and radial force components, relative to axis B. The distribution of the driving force between the axial and radial components depends on the stroke angle of the motor 100. The axial component tends to drive the drive plate 110 away from the barrel 104 along axis B, which is prevented by the thrust bearing 118. The radial component of the driving force tends to drive the socket of the drive plate 110, into which the second end of the piston 108 is seated, to move downward, causing the drive plate 110 to rotate so that the socket moves further away from the barrel, with the barrel 104 rotating in unison with the drive plate 110.
It will be recognized that the lower the stroke angle, the more of the driving force will be distributed to the drive plate 110 as an axial force, until, at a zero stroke angle such as that shown in FIG. 1C, all of the drive force is distributed to the drive plate 110 as an axial force. On the other hand, when the motor 100 is at a high stroke angle such as that shown in FIG. 1A, more of the drive force will be distributed radially and will be experienced by the bearing 118 as a radial force. Moreover, because the drive force is in a downward direction, as viewed in the figures, all of that radial force will be experienced by the lower part of the bearing 118. At the same time, the drive plate 110 and shaft 120 act as a lever, against the bearing 118 as a fulcrum, such that an upward radial force is exerted on the axial bearing 119.
When the motor is at zero stroke angle, as shown in FIG. 1C, cylinders 106 on one side of the barrel 104, divided down the line defined by TDC and BDC, are at high-pressure, while those on the opposite side are at low pressure. Thus, the thrust bearing experiences a very high axial load on one side, and a much lower axial load on the other. These high and low sides are separated by 90° from the high and low sides with respect to radial distribution. Furthermore, if the pressure of the fluid circuit that drives the motor is reversed while the motor is rotating forward, the motor switches to pump mode, and the distribution of the axial load is reversed, so that the bearing 118 experiences the high axial load on the opposite side.
The motor 100 shown in FIGS. 1A-1C is depicted as having cylinders directly opposite one another such that when one cylinder 106 is at TDC, another will be at BDC. This arrangement is pictured to provide a view of cylinders 106 at both TDC and BDC in the same figure. However, in practice, most hydraulic motors employ an odd number of cylinders, typically seven or nine. As a result, in a nine-cylinder motor the number of cylinders that are pressurized at high-pressure will cycle back and forth between four and five cylinders, nine times for each revolution of the cylinder. This means that the axial and radial loads on the motor bearings will also drop by 20% each time there are four pressurized cylinders, then back up by the same amount when there are five pressurized cylinders.
In typical applications, pump/motors of the type described here experience frequent changes in direction and speed. While it has been thought desirable to employ fluid bearings with pump/motors of this kind in order to improve efficiency and reduce wear, it has been found problematic, due to the complex nature of the changes in force and vector at play in these systems.
It can be seen that the bearings of the motor 100 are subjected to widely ranging forces. Changes from high to low stroke angle, then back to high, can occur very fast and very frequently. Rotation speed and direction varies, and the motor may stop frequently. Finally, because of the odd-number arrangement of the cylinder barrel, there is a constant 20% fluctuation of force as the barrel rotates. Because of these extreme conditions, little success has been shown using fluid bearings.
A more detailed discussion regarding the operation and structure of hydraulic pump/motors may be found in U.S. Pat. No. 7,014,429, issued Mar. 21, 2006, entitled HIGH-EFFICIENCY, LARGE ANGLE, VARIABLE DISPLACEMENT HYDRAULIC PUMP/MOTOR; and U.S. Patent Publication No. 2005/0193888 A1, published Sep. 8, 2005, entitled EFFICIENT PUMP/MOTOR WITH REDUCED ENERGY LOSS, which patent and published patent application are incorporated herein by reference, in their entirety.